In the manufacture of vinyl chloride polymers by polymerization in an aqueous medium, polymer deposits are formed on the inner walls of the polymerization autoclave and on the inserted devices. These deposits reduce the polymer yield and deteriorate the quality of the product, because part of these deposits coming off from the walls contaminate the final product by causing specks or fish-eyes. Furthermore, these deposits obstruct dissipation of the polymerization heat through the reactor walls, which results in unprofitably long reaction times.
Removal of such deposits is therefore absolutely required, which is usually carried out by mechanical means. Generally, pressurized water operated spray devices are employed for this purpose, which devices, however, remove the slightly adhering deposits only. Therefore, after every few batches, the reactor has to be cleaned manually with the trowel, and expensive safety measures have to be taken to protect the workers, so that these cleaning operations require much expenditure, cause long times of in-operation of the reactor and thus reduce considerably theprofitability of the manufacturing process.
Attempts have therefore been made to suppress or even avoid from the start such polymer deposits during the manufacture of vinyl chloride polymers in aqueous dispersion. A satisfactory solution of the problem, however, has not been found hitherto.
Some of the numerous processes described are based on the principle of reducing the formation of deposits by technological means; for example, scraping the walls of the reactor with a correspondingly shaped agitator; adjusting the wall temperature to at least the temperature of the reaction medium; cooling the reactor wall to -15.degree. to 0.degree. C.; feeding aqueous solutions, for example of salts of permanganic, chromic or dichromic acid, in the interface between the liquid and gaseous phases during the polymerization; polymerization while passing an electric current through the liquid reaction medium.
In other known processes, the components of the polymerization recipe are altered and/or further substance are added to the polymerization liquor.
Further known processes provide reactors with specially designed or coated inner walls in order to suppress the formation of deposits, for example walls having a roughness depth of less than 10 .mu.m, in addition to the use of water-soluble, reducing inorganic salts and adjustment of defined agitator speeds; or an insoluble wall coating of a cross-linked polymer material containing polar groups, which has been prepared using an aldehyde as cross-linking component; or a wall coating consisting substantially of polyethylene-imine hardened by means of an urea, aldehyde or diisocyanate, while optionally adding a bivalent tin salt of an inorganic acid as inhibitor to the polymerization medium; or a uniform wall coating containing at least one polar organic nitrogen, sulfur or oxygen compound of an anionic or nonionic wetting agent, an organic dyestuff or an inorganic pigment. There have been described furthermore wall coatings of polyaromatic amines.
None of these known processes for reducing or preventing deposits in the polymerization reactor is fully satisfactory, because each of the processes has one or more of the following disadvantages:
insufficient prevention of deposit formation; PA1 prolonged polymerization time, reduced yield; PA1 contamination of the polymer by clouding or dyeing substances and particles of the deposit on the walls which have altered due to the influence of heat and cannot be decomposed but with great difficulty; PA1 use of physiologically intolerable substances; PA1 undesirable alteration of the processing properties of the polymer (enlarged grain size, reduction of heat resistance); PA1 deteriorated activity on copolymerization and on use of certain initiators; PA1 necessity of expensive surface shaping or pretreatment such as polishing; PA1 insufficient reproducibility and limited modification possibilities of formulation and operation mode; PA1 more or less heavy influence of flow conditions on the processes; places of high turbulence being especially critical, therefore necessity of constantly renewed search for optimum conditions with each change of scale and proportions; PA1 special protection measures required for those reactor parts which are not in contact with the liquor, for example special spraying of dome and tubes; PA1 waste water problems due to additives to the liquor. PA1 R.sub.4 is R.sub.3 ; furthermore an O-organic hydrocarbon radical having from 1 to 12 carbon atoms, hydroxyl, halogen, amino, (di)-alkylamino having from 1 to 12 carbon atoms, carboxyl, sulfonyl; PA1 R.sub.5, R.sub.6 are R.sub.4, or PA1 R.sub.4 and R.sub.5 together are an aromatic ring. PA1 R.sub.1 is hydrogen, with the proviso that at least one, preferably two, of the radicals R.sub.4 to R.sub.6 must be carboxyl or SO.sub.3 H, and none of the remaining radicals is hydrogen, or none of the radicals R.sub.4 to R.sub.6 is hydrogen; PA1 R.sub.3 is hydrogen, a hydrocarbon radical of aliphatic character (alkyl, cycloalkyl, aralkyl) having from 1 to 12 carbon atoms, especially a linear or branched alkyl radical having from 1 to 6 carbon atoms, for example methyl, ethyl, propyl, butyl, i-butyl or n-hexyl; PA1 R.sub.4 is R.sub.3, or alkoxy having from 1 to 6 carbon atoms, such as methoxy, ethoxy, propoxy, hydroxyl, fluorine, chlorine, dialkylamino, carboxyl or sulfonyl; PA1 R.sub.5, R.sub.6 are R.sub.4 or PA1 R.sub.4 and R.sub.5 together are an aromatic ring having from 6 to 10 carbon atoms, especially a benzene ring optionally substituted by one or more functional groups as cited sub R.sub.3 /R.sub.4. PA1 1:1 copolymer of maleic anhydride+styrene PA1 1:1 copolymer of maleic anhydride+allyl acetate PA1 1:1 copolymer of maleic anhydride+vinyl acetate PA1 1:1 copolymer of maleic anhydride+vinylmethyl ether.
In accordance with a method not being included as yet in the published state of the art, there has been proposed to use certain benzothiazol-2-one-hydrazone derivatives as deposit-suppressing substances in the polymerization of vinyl chloride.